The present invention relates to a process for forming a protective coating on metals and metal alloy for use at high temperatures.
The use of diffusion coatings to protect metal parts operating at high temperatures from the destructive action of oxidation, corrosion, erosion, and thermal shock which form part of the normal operating conditions has been known for many years. Many types of such coatings have been proposed and, in fact, used and, in general, these contain aluminum as the main protective element. The life of these aluminum-based diffusion coatings depends not only on their thickness but above all on the resistance to rupture of the protective oxide layer which forms on the surface. With reference to the application of such coatings to the protection of turbine blades this resistance has become more and more critical with the continuing tendency to increase the operating temperatures of gas turbines.
The process generally used for the preparation of aluminum-based diffusion coatings is that of case hardening. This process consists of the immersion of a workpiece to be coated in a powder containing the protective material, an inert powder and an activator, then subjecting the whole, in a closed environment, to a heat treatment such as to induce the diffusion of the coating into the surface of the workpiece. This process has several disadvantages, the most important of which is the practical impossibility of coating the internal surfaces of cavities having very narrow access openings, such as the cavities of internally cooled turbine blades.
In addition, it is almost impossible to automate the case hardening process and difficulties arise due to the partial-coating of components of complex shape and wastage of material.
The object of the present invention is to provide a process for coating metals and/or metal alloys which is easy to carry out, of low cost and avoids at least some of the disadvantages referred to above. A further object of the invention is to provide aluminum based coatings on workpieces with greater resistance to high temperatures.